Many catalysts are known for the ring opening polymerization of epoxide monomers such as ethylene oxide. Examples of catalysts systems that are used for the industrial-scale production of poly(ethylene oxide) include calcium-based and zinc-based types of catalysts.
Alkylene oxide polymerizations employing a zinc-based catalyst are disclosed in the following references:
EP 0 239 973 A2 relates to zinc alkoxide and zinc aryloxide catalysts prepared from the reaction of a hydrocarbyl compound of zinc with a dispersion of a polyol in an inert medium. It is taught that the use of a dispersion aid such as fumed silica, magnesia or alumina and a nonionic solvent are critical to achieving good dispersion of the polyol in the inert medium. In this way fine catalyst particles are created. Preferred are linear polyols having from 2 to 6 carbon atoms in the alkane chain (most preferred having 4 carbon atoms) or a cycloalkane diol having 5 or 6 ring carbon atoms. Dispersion prepared catalysts are useful in the polymerization of cyclic alkylene oxides, e.g. ethylene oxide and propylene oxide, to produce high molecular weight polymers and copolymers.
U.S. Pat. No. 4,667,013 A describes as process for polymerizing alkylene oxides in the presence of a catalyst dispersion similar to that in EP 0 239 973 A2 above wherein a hydrogen-containing chain transfer agent having a pka value of from 9 to 22 is added to the polymerizing mixture to control the molecular weight of the resulting polymer. The chain transfer agent is preferably an alkanol (aliphatic alcohol) having from 1 to 16 carbon atoms.
U.S. Pat. No. 6,084,059 A details the preparation of metal alcoholate catalysts (including zinc alcoholates) wherein an organometallic compound is reacted with water or a active-hydrogen-containing compound such as an aliphatic polyol using a micelle or reversed-micelle technique facilitated by an ionic surfactant. The use of anionic surfactants is said to be most effective at promoting formation of micelles or reversed micelles which are subsequently reacted with the organometallic reagent such as diethylzinc to form an especially active catalyst. It is taught that the use of dispersion promoters such as fumed silica is not essential.
U.S. Pat. No. 5,326,852 A concerns the production of alkylene oxide polymers in the presence of a catalyst which is obtained by first reacting a hydrocarbyl compound of zinc with an aliphatic polyhydric alcohol, then reacting the product with a monohydric alcohol having 1 to 6 carbon atoms and finally applying a heat treatment at 80 to 200° C.
U.S. Pat. No. 6,979,722 B2 teaches the polymerization of an alkylene oxide in the presence of a catalyst in a branched aliphatic hydrocarbon solvent having 5 to 7 carbon atoms wherein the catalyst is a zinc compound obtained by the reaction of an organic zinc compound and an alcohol. In the example the catalyst is prepared by first reacting diethyl zinc with 1,4-butanediol and then with ethanol.
Catalyst systems for the alkylene oxide polymerization comprising Zn in combination with an additional metal are also known:
Polymer Letters, Vol 5, pp. 789-792 (1967) concerns bimetallic μ-oxo-alkoxides as catalysts for the polymerization of epoxides. One exemplary catalyst is Al4Zn2O5(OC4H9)6 which is used in the polymerization of propylene oxide. There is no mention of zinc alcoholates derived from polyols.
U.S. Pat. No. 3,607,785 A and DE 1 808 987 A describe the preparation of a catalyst by first reacting an Al alkoxide with Zn acetate and then contacting the resulting catalyst with a primary alcohol RCH2OH. There is no mention of zinc alcoholates derived from polyols. In the examples, the catalyst is used to polymerize propylene oxide.
U.S. Pat. No. 3,459,685 A teaches the polymerization of alkylene oxides with a catalyst system of a polymeric Al alcoholate and an organometallic compound, for example methyl zinc phenoxide is mentioned. There is no mention of zinc alcoholates derived from polyols.
U.S. Pat. No. 3,542,750 A is directed to the polymerization of alkylene oxides with a catalyst system of (a) the condensation product of Al hydroxide with an Al alcoholate and (b) an organometallic compound, for example methyl zinc phenoxide. There is no mention of zinc alcoholates derived from polyols.
DE 1 667 275 A and GB 1,197,986 A disclose a catalyst composition for the polymerization of alkylene oxide which composition comprises the reaction product of a partially hydrolyzed Al alkoxide and a group II or III organometallic compound. The organometallic compound is preferably diethyl zinc. There is no mention of zinc alcoholates derived from polyols.
DE 1 937 728 A and relates to a process for polymerizing alkylene oxide by contacting it with a catalyst prepared by reacting (1) an Al alkoxide with (2) phosphoric acid or an phosphoric acid monoester or diester, (3) an aliphatic alcohol and/or (4) a group II or III organometallic compound such as for example diethyl zinc. There is no mention of zinc alcoholates derived from polyols.
Zinc-based systems are also described as catalysts for the addition reaction of alkylene oxides with alkanols. U.S. Pat. No. 4,375,564 A is directed to the preparation of low molecular weight alkanol alkoxylates having 1 to 30 alkylene oxide units. The catalyst system employed is a combination of a first component of a soluble basic compound of Mg and a second component of a soluble basic compound of an element selected from various metals including Zn. The preferred Mg compounds are Mg alkoxides, preferably having 1 to 30 carbon atoms. The preferred second component is a metal alkoxide, preferably having 1 to 30 carbon atoms, more preferably 1 to 6 carbon atoms, most preferred 2 or 3 carbon atoms. Alcoholates derived from polyols are not mentioned.
The problem addressed by the present invention is to provide a new catalyst formulation that allows for the polymerization of epoxide monomers such as ethylene oxide to access a greater range of product polymer molecular weights including lower molecular weights than would be achievable with a zinc alkoxide catalyst alone.